The invention relates to integrated circuits and, more particularly, to band gap circuits for generating thermally stable voltage references.
In many integrated circuits, and particularly in those that control the functioning of power steps and protection stages, it is necessary to establish precise and thermally stable voltage references and/or current references that are not affected by the operating temperature of the integrated circuit. To provide such references, a band gap voltage reference followed by a current reference obtained using another band gap stage may be used. Referring to FIG. 1, a prior art band gap voltage reference stage is shown which produces a stable reference voltage expressed by:                     Vref        =                  Vbe1          +                      2            ⁢                          xe2x80x83                        ⁢                          R2              R1                        *            Vt            *            ln            ⁢                          xe2x80x83                        ⁢            10                                              (        1        )            
As the operating temperature increases, the base-emitter voltage Vbe1 decreases and the thermal voltage Vt increases, and the opposite occurs when the temperature decreases. Thus, a suitable ratio between the resistance values R2 and R1 may be selected to obtain a reference voltage that is substantially constant despite temperature variations.
If the currents I1 and I2 are identical, then:                     I1        =                  I2          =                                    Vt              R1                        *            ln            ⁢                          xe2x80x83                        ⁢            10                                              (        2        )            
Both Vt and R1 have a positive temperature variation when their coefficient is different because:                     Vt        =                                                            K                *                T                            q                        ⁢                          xe2x80x83                        ⁢            and            ⁢                          xe2x80x83                        ⁢                          R              ⁡                              (                T                )                                              =                      R            *                          (                              1                +                                  α                  *                  T                                            )                                                          (        3        )            
Accordingly, the current may either increase or decrease depending upon the operating temperature.
Therefore, in order to obtain a constant current reference, an additional precision stage is needed. The additional stage should generate a current substantially uneffected by temperature. A second band gap circuit may be used as a reference current stage for this purpose, as illustrated in FIG. 2. The thermally stable voltage Vref is used as a reference and is typically obtained through a dedicated stage such as the one shown in FIG. 1.
Equation (2) may be used to determine the reference current Iref where, by using different types of integrated resistors for Ra and Rb, the temperature coefficient of the denominator (R1) is made equal to that of the numerator (Vt). For example, one resistor may be implemented by a P+ diffusion where the other may be implemented by a Pxe2x88x92 diffusion.
Using the two prior art circuits described above requires two distinct precision stages to obtain both voltage and current references as well as double trimming, which in turn requires a substantial area of silicon in an integrated circuit. Accordingly, there is a need for reliable voltage references and current references that have simplified circuitry and that may be implemented using a single band gap stage.
It is therefore an object of the invention to provide both a voltage reference and a current reference which are substantially uneffected by operating temperature and that may be implemented using a single band gap stage.
This and other objects, features, and advantages of the present invention are provided by a circuit based on a band gap voltage reference stage including a pair of transistors having respective control nodes connected in common and a circuit for causing identical currents to pass through the transistors. An emitter (or source) of one of the transistors is coupled to ground through a first and a second resistor. An emitter (or source) of the other transistor (Q10) is coupled to ground through the second resistor. By adjusting the ratio between the two resistors, it is possible to establish a reference voltage (Vref) that is substantially constant despite varying temperatures on a base (or gate) common control node of the transistors.
The first resistor and the second resistor comprise two different types of integrated resistors. The first integrated resistor has a higher temperature coefficient than the temperature coefficient of the thermal voltage (Vt), and the second integrated resistor has a smaller temperature coefficient than the temperature coefficient of the thermal voltage (Vt).
An integrated circuit for generating a substantially thermally stable reference voltage and a substantially thermally stable reference current according to the invention is also provided. The integrated circuit includes an active load element providing a voltage, a start-up circuit coupled to a voltage reference for causing a bias current to pass through the active load element, and a control stage for controlling the start-up circuit. The control stage includes a first transistor coupled to a bias voltage and a second transistor coupled to the voltage of the active load element.
The integrated circuit also includes at least one band gap voltage reference stage for controlling the active load element. The at least one band gap voltage reference stage includes a pair of transistors and a current mirror coupled to the voltage of the active load element for causing substantially identical currents to pass through each of the pair of transistors. Additionally, a first output stage provides the substantially thermally stable reference voltage. The first output stage includes a high side transistor coupled to the voltage of the active load element and coupled to a supply voltage, and a low side a transistor coupled to the bias voltage. A second output stage converts the substantially thermally stable reference voltage into the substantially thermally stable reference current.